In recent decades non-vitreous inorganic or metal oxide fibers have become known in the art. Although such fibers have desirable physical properties, such as high temperature stability, their utility is inherently limited by their inability to withstand mechanical and chemical stresses accompanying certain applications. One such application is high temperature filtration.
A filtering apparatus having a multiplicity of tubular filter bags mounted in a filter housing is known in the art and commonly called a "baghouse". Each filter bag, generally made of gas permeable, woven inorganic or organic materials, has a tubular supporting frame or cage therein which holds the filter bag in an open tubular configuration. A gas stream laden with particulate matter flows into the bag and the particulate matter gradually becomes trapped or deposited on the exterior surface of the filter bag, due to the flow of gas from the outside to the inside of the filter bag. The particulate is not so trapped initially since the pores in the woven material are usually many times the size of the particulate separated. When the particulate laden gases first pass through the woven material or fabric, the efficiency of separation is low until enough particulate has been trapped to form a "precoat" on the fabric. With particulates frequently encountered in industrial processes, the precoat layer will normally form in a matter of seconds. Once the precoat layer is formed, the efficiency of separation of the particulate from the gas stream will usually be better than 99 percent, i.e., about 99 percent of the particulate is removed from the gas stream. With the passage of time, as a result of particulate accumulating in the fabric of the filter bag, the residual pressure drop across the filter bag gradually increases and the bags must be replaced, either due to excessive pressure drop or because of bag failure.
Efficiently operating woven fabric baghouse filters generally have a gas flow pressure drop of 2 to 6 inches (5 to 15 cm) of water and are rated at 1 to 8 cubic feet per minute per square foot (3.2 to 25.6 liters per minute per dm.sup.2) of fabric area. As the particulate layer builds up on the filter fabric, the pressure drop across the filter increases and it becomes necessary to clean the filter bag to dislodge the accumulated particulate therefrom. A manometer connected across the filter bag is used to determine when the filter should be cleaned. The cleaning can be automatically performed, either by mechanical shaking or reverse jet flow. The interval between cleanings can be from a few minutes to hours depending upon the rate of collection of particulate on the filter fabric. In a baghouse cleaning operation, where a section of filters can be removed from operation by means of automatic dampers, the cleaning operation can be performed in a matter of seconds, e.g., 2 to 10 seconds.
The presence of particulate within the filter bag fabric causes abrasive wear of the filter bags during cleaning and results in a shortened useful life. Any movement of the filter bag material, particularly adjacent to the stiffer sewn seams, and especially when loaded with an abrasive particulate, can increase the rate of the abrasive action, particularly on high modulus inorganic fibers which are inherently highly susceptible to abrasive wear. A support cage for the filter bag which has excessively large openings will permit undue flexing of the fibers in the filter fabric during cleaning. Thus, the basic form and structure of the filter bag and filter bag cage are critical factors in extending filter service life, especially when used for the removal of highly abrasive particulate in a corrosive gas, under which conditions a filter bag may wear out in a matter of weeks and need replacement.
The prior art has recognized the problem of cleaning bag filters and the problem of filter fabric wear by abrasion. U.S. Pat. No. 4,259,095 discloses support-diffuser tubes for improved support and particulate cleaning of filter bags which are subjected to cleaning by a pulse of reverse purge air. U.S. Pat. No. 4,149,863 discloses a fiberglass cloth bag filter over a wire mesh cage, the bag being cleaned by reverse blasts of air. Glass fiber and the mesh support are alleged to overcome problems of the art relating to synthetic fiber type bag filters failing at elevated temperatures in corrosive environments. U.S. Pat. No. 3,884,659 discloses a flexible, conventional, porous filter bag in slack position over a cylindrical wire cage, the bag being cleaned by a reverse jet air blast which causes the bag to inflate and snap away from the cage so as to dislodge accumulated solids from the filter media. U.S. Pat. No. 4,398,931 teaches a ceramic fabric tape tightly spiral-wound over a rigid frame to form a high temperature stable filter. The preparation of such a filter is labor-intensive, and the over-lap configuration results in the filter having areas of varying gas permeability.
Synthetic organic and glass fiber fabric bag filters have been used in the art in baghouse applications. It is known that many synthetic organic fabrics deteriorate at temperatures above 300.degree. C. and glass fiber deteriorates above 450.degree. C.
The present society's need for energy conservation has made it desirable to reclaim waste heat from power plants using fossil fuels. It is necessary to remove particulate matter from gas streams exiting from these plants, where the gases often are at a temperature in the range of 700.degree. to 1000.degree. C., before sending the hot, often corrosive, gas through an expansion or combustion turbine which is used for producing electrical energy. Fabrics used in prior art filters, such as glass fiber materials, cannot withstand these high temperatures in combination with corrosive sulfur combustion products of fossil fuels. Further, fabrics which can withstand the higher temperatures deteriorate due to the abrasive character of some particulate matter and the flexing of the fibers during filtration and cleaning of the filter by reverse air jet blasts.